The ability to replicate natural materials has substantially improved over the years. For example, decorative laminates have replaced natural materials in the construction of furniture, cabinets, counter tops, flooring panels and other surfaces. In each of these applications, a decorative surface may be applied to a core layer or substrate, namely, plywood, particle board, chipboard, medium density fiberboard, etc. Often, a backing layer is secured to the opposite surface of the substrate to balance the laminates or provide other benefits.
Generally, the decorative surface and the backer layer will include one or more kraft paper layers which are adhesively laminated together using various materials, such as melamine formaldehyde and phenolic resins. As shown in FIG. 1, a high pressure laminate 11 may comprise a very thin overlay sheet 12 impregnated with melamine resin, a decorative sheet 13 disposed thereunder, and sheets 14 and 15 of kraft paper impregnated with phenolic resin disposed below the decorative sheet.
The melamine impregnated overlay sheet 12 forms a hardened layer on the surface of the decorative sheet. This hardened layer of the decorative laminate is used to protect the surface of laminate 11, such as by making the laminate scratch and abrasion resistant. The melamine impregnated overlay also prevents discoloration or deformity of the laminate surface due to various external factors, such as high pressure and temperature and other ordinary stresses which occur in the environment where such laminates are typically used. Furthermore, the overlay sheet is also capable of easily withstanding the thermal or chemical strains occurring in these environments. For example, the melamine overlay sheet protects the laminate from discoloring when a very hot substance, such as tea or coffee, or a very cold substance, such as ice, spills on the surface of such a laminate. Thus, the melamine coating can withstand the very high and very low temperatures to which the laminate is exposed in everyday use.
However, the overlay sheet 12 itself may cause warping of the laminate panel under extreme hot, cold, or dry conditions. The conventional substrate or core layer 16 may not be able to withstand the pressure created by the movement of the melamine overlay of the decorative surface under these extreme conditions and may deform, delaminate, or in extreme cases, break due to the pressure exerted by the melamine overlay layer 12. Therefore, backer type laminates may be used for many applications, such as to provide balancing sheets on the bottom of decorative laminates. These backer laminates may comprise a discrete melamine impregnated balancing layer 19 to balance the melamine layer of the decorative surface, and sheets of phenolic resin impregnated kraft paper 17 and 18 to correspond to the layers in the decorative laminate. The balancing layer 19 used in prior art systems is similar to the overlay sheet 12 of the decorative surface and uses the same material as the overlay sheet. However, the balancing layer 19 may or may not be transparent as it is usually not visible. The discrete melamine impregnated balancing layer 19 when used in a backer laminate prevents warping of the laminate due to the movement of the melamine layer 12 of the decorative surface under extreme conditions.
Typically, in prior art systems, sheets of kraft paper are impregnated with phenolic resin by submerging them in a vat which is filled with phenolic resin and then curing the phenolic resin impregnated kraft paper. The kraft paper soaks up a desired amount of phenolic resin based on the time it is left in the vat and the level of submergence. This method of impregnating the kraft paper is generally not cost effective as it requires large vats providing substantial resin pool surface areas in order to allow the proper immersion of a portion of a continuous roll of kraft paper. These large pool surface areas result in wasteful use of phenolic resin as the large vat surface area is prone to collection of contaminants and to the escaping of resin vapors thus causing variations in the percentage of solids and/or other controlled attributes of the resin requiring substantial portions of the resin to be disposed of from time to time. Moreover, when the resin impregnated kraft paper is being manufactured using such vats, fumes are created during the process which are harmful to the workers in the vicinity of the manufacturing process. All of this is compounded by the fact that such vats of phenolic resin, or other resins, are difficult to clean requiring an inordinate amount of time to properly clean the vats that have been used for impregnating the kraft sheets with phenolic resin.
The melamine impregnated sheet used in the decorative and backer layers is usually not a kraft paper sheet but rather a very thin sheet specifically adapted, such as by controlling strand orientation, density, and porosity to carry the melamine resin. Kraft paper sheets are typically not suitable to act as a carrier for melamine because the porosity, strand orientation, and density of kraft paper sheets are not adapted for this purpose although they are well suited for phenolic resin impregnation. Typically, a suitable sheet is impregnated with melamine by coating both sides of the sheet with melamine formaldehyde resin and then removing excess resin from the sheet. The melamine formaldehyde coated sheet is cured under controlled conditions to produce the melamine impregnated sheet which may be used both in decorative and backer laminates.
The laminates which are manufactured by using the phenolic resin impregnated kraft paper sheets and the melamine impregnated sheets as described above are made by a bulky manufacturing press which is expensive to operate. Thus, it is not cost effective or desirable to use the press to produce individual laminates. Therefore, in the typical manufacturing process a plurality of laminates are produced from each press during each press cycle to make the most efficient use of the press.
Typically, in such a press system pairs of laminate assemblies, similar to the laminate assembly shown in FIG. 1, with or without pattern layer 13, are positioned back-to-back with the phenolic resin impregnated kraft paper sheet 15 of one laminate assembly facing the phenolic resin impregnated kraft paper sheet of a second laminate assembly with a discrete release sheet disposed there between as described below. These pairs of laminate assemblies are separated from other laminate assembly pairs by metal sheets or press plates. Usually during the manufacturing process each laminate assembly 11 also includes a sheet, known as release sheet. The release sheet is usually kraft paper which has been coated with a release agent on at least one side. This release sheet is placed at the end of each laminate assembly adjacent to the phenolic resin impregnated kraft paper layer 15 away from the press plates to provide a release mechanism between the paired laminate assemblies. The release sheet facilitates easy separation of the laminate assemblies after pressing as the release agent will not allow cross linking of the laminate assemblies at least with respect to the side of the laminate assembly to which the release sheet is applied.
Typically these laminate assemblies are subjected to a pressure and temperature for a time sufficiently long enough to cure the laminating resins impregnating the respective layers. The high temperatures and pressure cause the resins within the sheets to flow which consolidates the whole into an integral mass. Thus, typically the discrete melamine layer sticks to the phenolic resin impregnated kraft paper layer disposed adjacent to it due to the migration of the phenolic resin into at least a portion of the discrete melamine layer to provide structural cross linking. Accordingly, the melamine sheet generally adds to the thickness or bulk of the laminate.
The conventional laminates produced by the above described prior art systems may then be cut to size and employed in a variety of applications such as decorative surfaces for desktops, tabletops, wall panels, and the like such as by bonding them to a core layer or substrate with a conventional adhesive such as contact cement. These laminates may also be used as backer layers common in laminated flooring products.
When such a laminate is used as a decorative laminate, the melamine layer in the laminate may be used to protect the surface of the decorative laminate. As mentioned above, however, such a use of a melamine layer on a decorative side of a laminate may necessitate the use of a melamine layer on a backer to provide balancing. When such a laminate is used as a backer laminate, the melamine impregnated sheet acts mostly as a carrier for melamine in order to provide a melamine layer to counteract the stress created by the melamine sheet of the decorative laminate to prevent warping of the laminate.
The use of a discrete melamine sheet during pressing of laminate assemblies as described above presents certain disadvantages. The melamine sheet itself contributes substantially to the material cost of the manufactured laminate as the melamine sheet is generally more expensive than the kraft paper sheets. As discussed above, kraft paper is not a good carrier of melamine. Thus, a different kind of thin sheet is usually adapted to provide the necessary strand orientation, density, and porosity, to enable it to act as a carrier for melamine. This requires a complete additional processing step to provide a discrete sheet coated with melamine. Also, as the melamine sheet of prior art systems is very thin, it may easily be damaged during handling, resulting in substantial losses due to handling spoilage. Furthermore, as the melamine sheet is an additional sheet that has to be processed, there are substantial processing costs, such as handling and collating costs, scrap losses due to the brittle and difficult to handle nature of the melamine impregnated sheet, in addition to the costs associated with impregnating the sheet itself with melamine. Furthermore, the translucent character of the overlay sheet, although present due to the same type sheet being used for providing a decorative overlay being used for backer purposes, is not generally necessary for backer laminates.
Moreover, in order to achieve the desired thickness of the laminate assembly and still allow the use of a discrete melamine sheet, more sheets of thinner material instead of fewer sheets of thicker material have to be used. This increases the manufacturing overhead as a greater number of sheets have to be handled and processed before they can be used in the laminate assembly. Such handling and processing may include impregnating the sheets with phenolic resin, cutting the sheets to the desired size, and collating the sheets for subsequent pressing.
Furthermore, since the overlay sheet becomes part of the laminate after pressing, at least one sheet of the laminate does not include phenolic resin saturation to provide structural bonding, but rather relies on migration of the resin from an adjoining sheet. Thus, prior art systems do not provide consistent structural bonding between the different layers of the laminate as the phenolic resin saturation of at least one layer is less than the other layers. Therefore, the structural bond between different layers of the laminate assembly are not the same and may result in earlier delamination of a layer of the laminate.
It should be clear that the use of the melamine impregnated sheet contributes substantially to the cost of the manufactured laminate and also adds to the product cycle time. Not only are there raw material costs involved with the use of a separate melamine sheet, but also substantial undesirable processing costs are inherent with such a use.
Thus, there is a need in the art for a system and method of manufacturing laminates using the advantages offered by melamine impregnated sheets as a discrete sheet in laminates without introducing unnecessary costs, handling steps, or structural disadvantages attendant with the use of prior art discrete melamine sheets.